Synthesis, characterization, biological activity, and corrosion inhibition in acid medium of unsymmetrical tetradentate N2O2 Schiff base complexes

Article abstract of DOI:10.1002/jccs.201800027

Four divalent metal(II) complexes, namely [Co(II)L(H₂O)Cl]·2H₂O, [Ni(II)L(H₂O)Cl]·4H₂O, [Cu(II)L(H₂O)Cl]·3H₂O, and [Zn(II)L(H₂O)Cl]·5H₂O, {L = 2-furan-2-ylmethyleneamino-phenyl-iminomethylphenol}, were synthesized and characterized by several techniques. The molar conductance measurement of all analyzed complexes in DMSO showed their non-electrolytic nature. The new Schiff base ligand (HL) acts as tetradentate ligand, coordinated through deprotonated phenolic oxygen, furan ring oxygen, and two azomethine nitrogen atoms. The ligand field parameters were measured for the metal complexes, which were found to be in the range notified for an octahedral structure. The molecular structural parameters of the synthesized HL ligand and its related metal(II) complexes were calculated and correlated with the experimental parameters such as infrared (IR) data. The investigated ligand and metal complexes were screened for their in vitro antimicrobial activities against different types of fungal and bacterial strains. The resulting data confirmed the examined compounds as a highly promising bactericides and fungicides. The antitumor activities of all inspected compounds were evaluated against colon carcinoma (HCT-116) and mouse myelogenous leukemia carcinoma (M-NFS-60) cell lines. The inhibition effect of HL ligand and its isolated complexes on the corrosion carbon in the form of a rod of area 0.35 cm² in HCl was investigated by measuring the weight loss at 25 °C.

Full text of DOI:10.1002/jccs.201800027

Received: 31 January 2018
Revised: 22 March 2018
Accepted: 25 March 2018
DOI: 10.1002/jccs.201800027
A R T I C L E
Synthesis, characterization, biological activity, and corrosion
inhibition in acid medium of unsymmetrical tetradentate
N2O2 Schiff base complexes
Amani S. Alturiqi1 | Abdel-Nasser M.A. Alaghaz2,3 | Mohamed E. Zayed4 | Reda A. Ammar5,6
1Department of Chemistry, College of Science,
Four divalent metal(II) complexes, namely [Co(II)L(H2O)Cl]ꢀ2H2O, [Ni(II)L(H2O)
Princess Nourah bint Abdul Rahman University,
Riyadh, Saudi Arabia
2Department of Chemistry, Faculty of Science,
Cl]ꢀ4H2O, [Cu(II)L(H2O)Cl]ꢀ3H2O, and [Zn(II)L(H2O)Cl]ꢀ5H2O, {L = 2-furan-
2-ylmethyleneamino-phenyl-iminomethylphenol}, were synthesized and character-
ized by several techniques. The molar conductance measurement of all analyzed
complexes in DMSO showed their non-electrolytic nature. The new Schiff base
Jazan University, Jazan, Saudi Arabia
3Department of Chemistry, Faculty of Science
(Boys), Al-Azhar University, Cairo, Egypt
ligand (HL) acts as tetradentate ligand, coordinated through deprotonated phenolic
oxygen, furan ring oxygen, and two azomethine nitrogen atoms. The ligand field
parameters were measured for the metal complexes, which were found to be in the
range notified for an octahedral structure. The molecular structural parameters of
the synthesized HL ligand and its related metal(II) complexes were calculated and
correlated with the experimental parameters such as infrared (IR) data. The investi-
gated ligand and metal complexes were screened for their in vitro antimicrobial
activities against different types of fungal and bacterial strains. The resulting data
confirmed the examined compounds as a highly promising bactericides and fungi-
cides. The antitumor activities of all inspected compounds were evaluated against
colon carcinoma (HCT-116) and mouse myelogenous leukemia carcinoma (M-
NFS-60) cell lines. The inhibition effect of HL ligand and its isolated complexes
on the corrosion carbon in the form of a ꢁrod of area 0.35 cm2 in HCl was investi-
gated by measuring the weight loss at 25 C.
4Department of Botany and Microbiology, Faculty
of Science, King Saud University, Riyadh, Saudi
Arabia
5Department of Chemistry, College of Science, Al-
Imam Mohammad Ibn Saud Islamic University
(IMSIU), Riyadh, Saudi Arabia
6Department of Chemistry, Faculty of Science
(Girls), Al-Azhar University, Cairo, Egypt
Correspondence
Abdel-Nasser M.A. Alaghaz, Department of
Chemistry, Faculty of Science, Jazan University,
Jazan, Saudi Arabia.
Email: aalajhaz@hotmail.com
Funding information
Deanship of Scientific Research at Princess Nourah
bint Abdulrahman University, Grant/Award
Number: RGP-1438-00
KEYWORDS
antimicrobial, antitumor, DFT, furan-Schiff base complex, spectroscopic data
have been able to provide diverse possibilities in the struc-
ture and chemical properties, and these complexes are the
1
| INTRODUCTION
Schiff bases have played a major role in the development of
coordination chemistry, and the synthesis of Schiff bases is
one the main activities in the areas of biochemistry, materials
science, hydrometallurgy, catalysis, activation, transport and
separation phenomena, etc.[1,2] Unsymmetrical Schiff base
ligands with hard and soft atoms have been widely investi-
gated. They readily form stable complexes with most of the
transition metals, and the synthesis of these complexes is an
important area of study with implications in bioinorganic
chemistry,[3–5] catalysis,[6] and medicinal chemistry.[7]
Unsymmetrical Schiff base ligands with N, O, and S donors
most abundant and dominate the field. The presence of both
hard and soft donor atoms in one ligand increases the coordi-
nation ability toward hard as well as soft metals.[8–11] The
interest in the coordination chemistry of tridentate unsym-
metrical Schiff base ligands with NOS donating sites has
increased because of their capability of forming stable com-
plexes of four, five, and six coordination[12,13] and because
they show a broad range of antifungal,[14] antibacterial,[15]
and anticancer properties.[16] Tridentate NOS donor unsym-
metrical Schiff base ligands have been able to form 1:1
and/or 1:2 complexes. In all complexes, the Schiff base
© 2018 The Chemical Society Located in Taipei & Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
J Chin Chem Soc. 2018;1–15.
http://www.jccs.wiley-vch.de
1

2
ALTURIQI ET AL.
calculated using the relation μeff = 2.828(χmT)1/2 BM, where
χm is the molar magnetic susceptibility corrected for the dia-
magnetism of all atoms in the compounds using Selwood and
Pascal's constants. Thermal analysis (TG) was effected by
using a Shimadzu TG-50 thermal analyzer at the heating rate
OH
O
NH2
NH2
+
O
O
+
ꢁ
of 10 C /min in nitrogen atmosphere at the rate 20 mL/min
ꢁ
in the temperature range 30–1100 C using platinum cruci-
bles. Powder XRD patterns were recorded with an X'Pert
PRO diffractometer using Cu Kα1 radiation (λ = 1.54060 Å)
at the operating voltage of 40 kV and current of 30 mA.
Thermal studies were conducted using the Q 600SDT and Q
20 DSC thermal analyzers. The biological activity experi-
ments were conducted at the Bab-Al-Sheria University Hospi-
tal (Al-Azhar Microbiology Laboratory University).
O
O
H
H
N
N
N
N
O
O
2.2 | Corrosion test by weight loss measurement
Enol form
Keto form
Six parallel steel sheets of dimensions 2.0 × 2.0 × 0.1 cm
were abraded with emery paper (grade to 1200 grit size) and
then washed with double-distilled water and acetone. After
accurate weighing, the specimens were immersed in a
250-mL beaker containing 100 mL of HCl with and without
the addition of different concentrations of the ligand and
metal complexes. All the aggressive acid solutions were
open to air, and the measurements were carried out at room
temperature (25 ꢂ1 ꢁC). After 2 hr, the specimens were
taken out, washed, dried, and weighted again accurately.
The experiment was repeated, and the average weight loss of
the six the parallel steel sheets was obtained. The inhibition
efficiency (%IE) of the investigated compounds for the cor-
rosion of steel in HCl was calculated as follows:
FIGURE 1 Synthesis and proposed structure of the ligand (HL)
ligand behaves as an NOS-tridentate ligand or an NS (with soft
metal ions) and/or NO-bidentate (with hard metal ions) ligand.
The most plausible geometry for the 1:2 complexes or the 1:1
complexes appears to be square-planar (four-coordinated),
trigonal-bipyramidal (five-coordinated), or octahedral (six-
coordinated).[12] In this paper, we describe the synthesis and
characterization of Co(II), Ni(II), Cu(II), and Zn(II) complexes
with the furan-Schiff base ligand (HL) (Figure 1). The possible
geometries for the related furan-Schiff base metal(II) com-
plexes were deduced by elemental analysis, molar conduc-
tance, magnetic susceptibility, infrared (IR), electronic and
thermal analysis, X-ray diffraction (XRD), electron paramag-
netic resonance (EPR), and molecular modeling studies.
%IE = ðRo −Ri=RoÞ × 100
ð1Þ
where Ro and Ri are the rates of reaction in the absence and
presence of the inhibitor, respectively. The experiment was
repeated at 2, 4, and 8 hr. The corrosion inhibition of the
compounds was tested on carbon in the form of a rod of area
0.35 cm2. Steel has the composition (in wt%) 0.18 C, 0.06
Si, and the rest Fe, 0.95 Mn, 0.013 Ni, 0.009 P, 0.004 S, 0.6
Si, 0.008 Cr, 0.033 Al, 0.16 V, 0.004 Ti, and 0.00 Cu. The
method of weight loss wasꢁused to study the corrosion inhi-
bition of carbon steel at 25 C.
2
| EXPERIMENTAL
2.1 | Materials and methods
Elemental analyses (C, H, N) were carried out on a Perkin–
Elmer 2,408 CHN analyzer. Electronic spectra were recorded
on a UV2 Unicam UV–vis spectrophotometer. Magnetic
moments (μeff) were measured with a Sherwood scientific
magnetic susceptibility balance at 294 K. IR spectra were
recorded on a Mattson 5000 Fourier transform infrared
2.3 | Synthesis of the Schiff base ligand
1
The new furan-Schiff base ligand (HL) was prepared
(FTIR) spectrophotometer. H NMR measurements at room
by adding benzene 1,2-diamine (0.108 g,
1 mmol),
temperature were carried out on a Varian Gemini 200 MHz
spectrometer, a Varian-Mercury VX-300 NMR spectrometer,
and a Brucker 500 (Avance iii) instrument. Molar conductivi-
ties were measured in DMSO solution for the complexes
(10−3 M) using a CON 6000 conductivity meter (Cyberscan,
Eutech instruments). Magnetic susceptibilities of the com-
plexes were calculated by the modified Gouy method at room
temperature using a magnetic susceptibility balance (Johnson
Matthey, UK). The effective magnetic moments were
2-hydroxybenzaldehyde (0.122 g, 1 mmol), and furan-
2-carbaldehyde (0.096 g, 1 mmol) and dissolving them in
dry absolute ethanol (40 mL). The reaction mixture was
heated under reflux for 3 hr, and a yellow precipitate was
formed upon cooling the solution to room temperature. The
product was filtered off and washed with a small quantity of
ethanol and then diethyl ether, air-dried, and recrystallized
from ethanol. The purity of the prepared furan-Schiff base

ALTURIQI ET AL.
3
TABLE 1 Formula, molecular weight, melting point, yield, and elemental analysis data of the complexes
Elemental analysis calc. (found)
M.P.
(ꢁC)
Yield
(%)
a
No.
Ligand
1
Compound M.F. (M.W.)
HL C18H14N2O2 (290.3)
[CoL(H2O)Cl].2H2O
Color
Orange
Olive
C
H
N
Cl
M
Λm
142
93
86
74.47 (74.68)
49.39 (49.39)
4.86 (4.73)
4.38 (4.38)
9.65 (9.52)
6.40 (6.37)
—
—
—
>300
8.10 (8.04)
13.46 (13.42)
7.32
9.12
6.54
8.36
C18H19ClN2O5Co (437.7)
2
3
4
[NiL(H2O)Cl].4H2O
C18H23ClN2O7Ni (473.5)
Brown
>300
>300
>300
84
83
86
45.66 (45.53)
46.96 (46.91)
43.39 (43.33)
4.90 (4.88)
4.60 (4.55)
5.06 (5.00)
5.92 (5.82)
6.09 (6.02)
5.62 (5.57)
7.49 (7.43)
7.70 (7.68)
7.12 (7.08)
12.39 (12.34)
13.80 (13.76)
13.12 (13.09)
[CuL(H2O)Cl].3H2O
Pale brown
Orange
C
18H21ClN2O6Cu (460.4)
[ZnL(H2O)Cl].5H2O
18H25ClN2O8Zn (498.2)
C
M.P. melting point; M.F. = molecular formula; M.W. = molecular weight.
a
Λ
m = molar conductance (Ω−1 cm2 mol−1).
ligand was measured by thin-layer chromatography (TLC)
employing silica gel, as shown in (Figure 1).
3
|
RESULTS AND DISCUSSION
3.1 | Characterization of the Schiff base ligand (HL)
2.4 | Synthesis of the metal(II) Schiff base complexes
3.1.1
| Elemental analyses
The new furan-Schiff base (HL) ligand (1 mmol) was dissolved
in 10 mL of dichloromethane, then 1 mmol of CoCl2ꢀ6H2O,
NiCl2ꢀ6H2O, CuCl2ꢀ2H2O, or ZnCl2 dissolved in 10 mL of
ethanol was added dropwise, and the reaction mixture was stir-
The novel azomethine furan-Schiff base HL ligand was pre-
pared and subjected to elemental, mass, and IR spectral ana-
lyses. Results of the microanalytical data (C, H, N), along
with the molecular formula (M.F.) and the melting point
(M.P.), are given in Table 1. The results obtained are in good
agreement with those calculated for the suggested formula,
and the melting point is sharp, indicating the purity of the
prepared azomethine furan-Schiff base HL ligand. The struc-
ture of the azomethine furan-Schiff base HL ligand under
study is shown in Figure 1.
ꢁ
red and heated at 50 C for 4 hr. The solvent was allowed to
evaporate slowly to give a solid. The resulting product was col-
lected by filtration and washed with dichloromethane and ethyl
acetate to get the pure complex. The physical properties and
analytical data of the new furan-Schiff base ligand and its cor-
responding metal(II) complexes are given in Table 1.
3.1.2
| IR spectrum
2.5 | Molecular modeling
Calculations using the DMOL3 program were performed in
the Materials Studio package,[17] which is designed for the
realization of large-scale density functional theory (DFT) cal-
culations. DFT semi-core pseudo-pods (dspp) calculations
were performed with the double numerical basis sets plus
polarization functional (DNP). The DNP basis sets are com-
parable in quality to the 6–31G Gaussian basis sets.[18] Del-
ley et al. showed that the DNP basis sets are more accurate
than Gaussian basis sets of the same size.[19] The RPBE
The IR spectrum (Table 2) of furan-Schiff base HL ligand is
given in “synthetic procedures”. Vibration bands at
3,364 cm−1 (νO–H), 3,076 cm−1 (νC–H, Ar–H),
1,623 cm−1 (νCH=N), 1,567 cm−1 (νC=C), and 1,214 cm−1
(νC–O, Ar–O) were observed for the azomethine Schiff base
ligand (HL) (Figure S1, Supporting information). The
stretching frequency at 2853 cm−1 in the furan-Schiff base
HL ligand indicates the presence of the O–HꢀꢀꢀN intramolec-
ular hydrogen bond. The HL ligand with a strong band at
1,214 cm−1 possesses highest percentage of enolimino tauto-
mer because of the stabilization of the phenolic C–O bond.
Similarly, the band at 1,024 cm−1 in the HL ligand corre-
sponds to the ν(C–O–C) stretching vibration of the furan
ring.[22]
functional[20]
,
so far the best exchange-correlation
functional,[21] based on the generalized gradient approxima-
tion (GGA), was employed to take into account the exchange
and correlation effects of electrons. Geometric optimization
was performed without any symmetry restriction.
TABLE 2 IR spectral data (cm−1) of the furan-Schiff's base HL ligand and its metal complexes
Compound
νOH
3364br
—
ν(OH)water
—
νC=N
νC–O
ν(C–O–C)furan
1,024 m
1,008 m
1,009 s
νM–O
—
νM–N
—
νM–Cl
—
HL
1,623 m
1,601 m
1,600 m
1,601 s
1,600 m
1,214 m
1,202 m
1,200 m
1,203 m
1,203 m
[CoL(H2O)Cl].2H2O
[NiL(H2O)Cl].4H2O
[CuL(H2O)Cl].3H2O
[ZnL(H2O)Cl].5H2O
3318 br
3320 br
3320 br
3300 br
376 m
380 s
363 m
365 m
355 w
348 w
352 m
345 m
262m
274 w
267 m
277 m
—
—
1,012 m
1,010 m
br = broad; m = medium; s = small; sh = sharp; w = weak.

4
ALTURIQI ET AL.
148, 152, 208, and 227 u, are attributable to different frag-
ments of the furan-Schiff's base HL ligand. These data sug-
gest the condensation of the keto (–C=O) group with the
amino (–NH2) group. The molecular ion peak of azomethine
furan-Schiff base HL ligand (290 u) is in good agreement
with the proposed molecular formula indicated from elemen-
tal analyses.
3.4 | Characterization of the furan-Schiff base metal
complexes
On the basis of analytical data (Table 1), all the furan-Schiff
base metal(II) complexes have the general composition [ML
(H2O)Cl].nH2O {L = 2-furan-2-ylmethyleneaminophenyl-
iminomethyl-phenol and for M = Co(II), n = 2; Ni(II),
n = 4; Cu(II), n = 3; and Zn(II), n = 5}. The obtained furan-
Schiff base complexes are ꢁpowders, which are stable in air
and decompose above 300 C (Table 1). They are soluble in
organic solvents such as acetone, chloroform, methanol,
DMF, and DMSO but insoluble in most nonpolar solvents.
The Λm (molar conductance) values of the soluble related
furan-Schiff's base metal(II) complexes (1–4) in dimethylfor-
FIGURE 2 (a) 1H NMR spectrum of furan-Schiff base HL ligand. (b) 1H
NMR spectrum of the [ZnL(H2O)cl].5H2O complex
3.1.3
| NMR spectra
1
mamide (DMF) are in the range 6.54–9.12 Ω−1 cm2 mol−1
,
The H NMR data of the HL ligand (Figure 2a) shows that
the tautomeric equilibrium favors the enol–imine in DMSO.
The broad signal at δ = 11.93 ppm is assigned to the proton
of the hydroxyl group. This signal is due to the hydrogen-
bonded phenolic proton, and the integration is generally less
than 2.0 due to this intramolecular hydrogen bonding. The
signal due to the azomethine (CH=N) proton of the character-
istic azomethine group for azomethine Schiff base –N=C(H)–
was observed at 8.57 ppm. In the region 6.88–7.37 ppm, the
chemical shifts were assigned to the hydrogen of the aromatic
ring. In the 13C NMR spectrum of the HL ligand (Figure S2),
azomethine carbon is at 160.12 ppm.
indicating their non-electrolyte nature.[23]
3.5 | IR spectra
In the IR spectra of the furan-Schiff base HL ligand
(Figure S1; Table 2), the absorption at 1623 cm−1 is attrib-
uted to the presence of the azomethine group (–CH=N). The
shifting of this band to lower frequencies (by 23 cm−1) in all
the corresponding furan-Schiff base metal(II) complexes
indicate the coordination of the azomethine nitrogen (–C=N)
to the metal ion, which can be explained by the donation of
electrons from the nitrogen to the empty d orbital of the
metal ion. A broad band at 3364 cm−1 is observed, which is
due to the phenolic OH group, and disappears in the spectra
of all furan-Schiff base metal(II) complexes, suggesting the
deprotonation of the phenolic OH group by the metal ion.
The band observed at 1214 cm−1 is assigned to phenolic
C–O stretching vibration, which gets shifted to lower fre-
quencies in all the corresponding furan-Schiff base metal(II)
complexes and is consistent with coordination via the pro-
tonated phenolic oxygen.[24] Also, the specific band of the
ν(C–O–C) furan ring vibration at 1024 cm−1 in the ligand
shifts to 1,008–1,012 cm−1 in all furan-Schiff base metal(II)
complexes.[24] The bands observed in the ranges 380–363,
355–345, and 262–277 may be assigned to (M–O), (M–N),
and (M–Cl), respectively.[24]
3.2 | UV spectrum
The UV spectra of the azomethine furan-Schiff base HL
ligand in ethanol, dichloromethane, acetonitrile, and hexane
show absorption bands at 224, 232, 257, and 262 nm, which
are assigned to the π !π* transitions of the enol–imine tau-
tomer of the ligand. The observed small hypsochromic shift
of the 332 nm band in more polar solvents is typical of the
n!π* transitions of the C=N group. The maximum at
417 nm detected in EtOH solutions of the furan-Schiff base
HL ligand is assigned to the n!π* transitions in the dipolar
zwitterionic or the keto–imine tautomeric structures.
3.3 | Mass spectrum
3.5.1
| NMR spectral data
The electron impact (EI) mass spectrum (Figure S3) of the
free azomethine furan-Schiff base HL ligand confirms the
proposed formula by showing a peak at 280 u corresponding
to the ligand moiety [(C18H14N2O2) atomic mass 290 u].
The series of peaks in this range, namely 52, 82,118,
The 1H NMR spectroscopic data of the furan-Schiff base HL
ligand (Figure 2a) and its [ZnL(H2O)Cl]ꢀ5H2O complex
(Figure 2b) were collected in DMSO-d6. The broad signal at
11.93 ppm is due to the –OH proton in the furan-Schiff's

ALTURIQI ET AL.
5
FIGURE 3 Electronic spectra of the (a) [CoL(H2O)cl]ꢀ2H2O complex, (b) [NiL(H2O)Cl]ꢀ4H2O complex, (c) [CuL(H2O)]ꢀ3H2O complex, and (d) [ZnL
(H2O)].5H2O complex
base HL ligand, and the disappearance of the signal due to
the –OH proton in the spectrum of the [ZnL(H2O)]ꢀ5H2O
complex supported the protonation of the thiol group. The sig-
nal of the azomethine proton is shifted to high field in the spec-
trum of the [ZnL(H2O)Cl].5H2O complex. It appears at
9.88 ppm as compared to 8.57 ppm in the furan-Schiff base HL
ligand. This indicates complexation to the zinc atom through the
azomethine nitrogen atom. The 1H NMR spectrum of the furan-
Schiff base HL ligand shows a multiplet at 7.24–7.73 ppm due
to aromatic protons. The signal observed at 3.13 ppm with an
integration corresponding to 12 protons in the case of [ZnL
(H2O)Cl]ꢀ5H2O complex was assigned one coordinate and five
1
hydrate and one coordinate water molecules. The H NMR
spectra of the furan-Schiff base HL ligand and of the [ZnL
(H2O)Cl]ꢀ5H2O complex are given in Figure 2a and b.
In the 13C NMR of the furan-Schiff base HL ligand
(Figure S2), the (–C=N–) azomethine carbon showed a sig-
nal at 160.12 ppm,[25] the (–C–O) phenolic group carbon at
163.27 ppm,[26] and the (C–O) furan ring at 170.12.[27] The
signals due to (–C=N–) azomethine, (–C–O) phenolic, and
TABLE 3 Molar conductance, magnetic moment, and electronic spectral data of the complexes
Complex
Geometry
μeff (BM)
Band assignments
λ )
max (cm−1
4
[Co(L)(H2O)Cl].2H2O
Octahedral
3.98
4T1 ! T2(F) (ν1)
10,696
15,191
25,067
11,357
15,803
21,489
18,534
30,373
25,674
4
4T1 ! A2(F) (ν2)
4
4T1 ! T1g(P) (ν1)
3
[Ni(L)(H2O)Cl].4H2O
Terahedral
2.92
3A2(F) ! T2(F) (ν2)
3A2(F) ! T1(F) (ν3)
3
3A2(F) ! T1(P) (ν3)
3
2
[Cu(L)(H2O)Cl].3H2O
[Zn(L)(H2O)Cl].5H2O
Octahedral
Octahedral
2.06
2B1g ! A1g(dx2 − y2 ! dz2)(ν1)
2B1g ! A2g(dx2 − y2 ! dzy)(ν2)
2
Diamagnetic
LMCT(M N)

6
ALTURIQI ET AL.
TABLE 4 Ligand field parameters of the complexes
Complex
Dq (cm−1
1168
792
)
B (cm−1
808
)
β
LFSE (kJ mol−1
100.47
)
F2
—
—
F4
—
—
C
ν2/ν1
1.42
1.39
2.00
[Co(L)(H2O)Cl].2H2O
[Ni(L)(H2O)Cl].4H2O
[Cu(L)(H2O)Cl].3H2O
0.73
0.95
—
—
—
—
114.23
—
—
(C–O) furan carbons were slightly shifted downfield in com-
parison to the corresponding signals of these groups in the
ligand, thereby confirming the complexation with the zinc
metal ion. It was observed that DMSO did not have any
coordinating effect on the spectrum of the [ZnL(H2O)Cl]ꢀ
5H2O complex (Figure S2). The 1H and 13C NMR spectra of
the furan-Schiff base HL ligand and [Zn(L)(H2O)]ꢀ5H2O
mononuclear complex are given in Figures 3 and S2.
the [C5H9NOCo]+, [C5H10ONi]+, [C3H5OCu]+, and
[C3H11O2Zn]+ fragments, respectively.
3.5.3
| Electronic, ligand field parameters, and magnetic
susceptibility measurements
The electronic spectrum of the Co(II) complex (Figure 3a;
Table 3) shows three bands at 10,696, 15,191, and
25,067 cm−1 . These bands may be assigned to following
4
4
transitions: 4T1g ! T2g (F) (ν1), 4T1g ! A2g (F) (ν2), and
4T1g ! 4T1g(P) (ν3), respectively. The positions of the
bands suggest octahedral geometry of the Co(II) com-
plex.[28] The Co(II) complex shows a magnetic moment of
3.98 BM, corresponding to the three unpaired electrons. The
ligand field parameters were calculated for the cobalt(II)
complex. The value of Dq was calculated from Orgel energy
level diagrams using the ν3/ν1 ratio.[29] The value for B (free
ion) is 1,126 cm−1. The value of β indicates that the covalent
character of metal ligand σ-bond is low (Table 4).
3.5.2
| Mass spectra
The mass spectra of the furan-Schiff base metal(II) complexes
(Figure S3) and the molecular ion peaks at 437, 473, 460, and
498 for corresponding furan-Schiff base metal(II) complexes
were used to confirm the proposed formula (437, 473,
460, and 498u for Co(II), Ni(II), Cu(II), and Zn(II) complexes,
respectively). The multipeak pattern of the mass spectrum
gives an impression of the successive degradation of the target
compound, with the series of peaks corresponding to the vari-
ous fragments. Their intensity gives an idea of the stability of
the fragments. The last four fragments in all the complexes
appear at positions (m/z value) 158 (100%), 144 (<100%),
120 (<100%), and 144 (<100%). These values correspond to
The electronic spectrum of the Ni(II) complex
(Figure 3b; Table 3) displays bands at 11,357, 15,803, and
3
3
21,489 cm−1, and may be assigned to the A2g(F) ! T2g
(F) (ν1), 3A2g(F) ! 3T1g(F) (ν2), and 3A2g(F) ! 3T1g
(P) (ν3) transitions, respectively. It suggests the octahedral
geometry of the Ni(II) complex.[29] The nickel(II) complex
shows a magnetic moment 2.92 BM, corresponding to two
unpaired electrons.[30]
Electronic spectrum of the copper(II) complex
(Figure 3c; Table 3) displays bands at 14,992, 18,534 and
30,373 cm−1. The first two bands may be assigned to the
2
2
2
transitions B1g ! A1g (dx2 − y2 ! dz2) (ν1) and B1g
! B2g (dx2 − y2 ! dzy) (ν2), respectively, and third band
2
may be due to charge transfer. The observed magnetic
moment of the Cu(II) complex is 1.96 BM, which confirms
the octahedral structure of this complex.[30]
The Zn(II) complex (Figure 3d; Table 3) is diamagnetic
as expected, and its geometry is most probably octahedral
similar to the Cu(II), Co(II), and Ni(II) complexes of the HL
ligand.
3.5.4
|
EPR spectrum of [CuL(H2O)Cl]ꢀ3H2O complex
The solid-state ESR spectrum of the [CuL(H2O)Cl].3H2O
complex (Figure 4a, Table 5) was recorded at room tempera-
ture. The shape of the spectrum is consistent with the
TABLE 5 EPR bonding parameters of Cu(II) complex at room
temperature
α2
β2
γ2
G
K||
K⊥
FIGURE 4 (a) EPR spectrum of the [Cu(L)(OH2)]ꢀ3H2O complex.
(b) TGA spectra of the furan-Schiff's base ligand (HL) and its metal
complexes (1–4)
Complex
[Cu(L)(H2O)Cl].3H2O 0.7235 0.7365 0.6287 4.63 0.7369 0.2348

ALTURIQI ET AL.
7
TABLE 6 Crystallographic data for the Schiff base complexes [CoLCl(H2O)].2H2O, [NiLCl(H2O)].4H2O, [CuLCl(H2O)].3H2O, and [ZnLCl(H2O)].5H2O
Data
[CoLCl(H2O)].2H2O
C18H19ClN2O5Co
437.7
[NiLCl(H2O)].4H2O
C18H24ClN2O7Ni
473.5
[CuLCl(H2O)].3H2O
C18H21ClN2O6Cu
460.4
[ZnLCl(H2O)].5H2O
C18H25ClN2O8Zn
498.2
Empirical formula
Formula weight (g/mol)
Wavelength (Å)
Crystal system
Space group
a (Å)
0.71073
0.71073
1.54056
Orthorhombic
Pbca
1.54056
Triclinic
Monoclinic
P2(1)/c
Triclinic
Pī
Pī
5.6934(3)
9.3416(5)
12.6415(7)
116.513(6)
92.160(5)
102.669(5)
3,266.2(4)
1.557
9.2066(7)
19.1090(13)
18.589(2)
90
12.287 (5)
11.884 (5)
37.152 (5)
90
7.9981(5)
12.6314(8)
19.4346(10)
91.140(4)
96.019(4)
113.464(5)
856.46
b (Å)
c (Å)
α (ꢁ)
β (ꢁ)
γ (ꢁ)
100.46(3)
90
90
90
Volume (Å3)
(Calc.) density (g/cm−3
3,266.2(4)
1.557
5,425
)
1.543
1.701
2θ range
1.71–28.27
−16 ≤ h ≥ 6
−15 ≤ k ≥ 5
−15 ≤ l ≥ 5
2
1.71–28.27
−6 ≤ h ≥ 7
−34 ≤ k ≤ 34
−12 ≤ l ≥ 9
4
10.25–67.27
0 ≤ h ≥ 15
0 ≤ k ≤ 15
0 ≤ l ≥ 47
8
12.56–68.00
−10 ≤ h ≥ 10
−12 ≤ k ≤ 12
−25 ≤ l ≥ 25
2
Limiting indices
Z
Number of reflections
14,623
14,354
14,258
13,567
Number of independent reflections/parameters
Rint
164/2814
0.0266
323/2537
0.0224
332/2756
0.078
342/2428
1.004
Goodness-of-fit S
0.893
1.007
0.985
0.858
R (all data)/R[F02 > 4σ(F02)]
0.0341/0.0222
0.048
0.0362/0.0213
0.039
0.0290/0.0219
0.0322
0.0479/0.0238
0.0723
wR2 (all data)
Temperature (K)
298
298
298
298
octahedral environment around Cu(II) ion, and the higher g-
value of the investigated [CuL(H2O)Cl]ꢀ3H2O complex,
when compared to that of a free electron (g = 2.24), reveals
an appreciable covalency of the metal ligand bonding with
dx2–y2 as the ground-state characteristic of octahedral stereo-
chemistry.[31] Also, the g||/A|| value of 143 for the [CuL
(H2O)Cl]ꢀ3H2O complex lies just within the range expected
for the octahedral complex.[31] The decrease of the g-value
TABLE 7 Thermal analyses data for furan-Schiff's base ligand (HL) and the corresponding metal complexes (1–4)
Compound
Temp. range (ꢁC)
50–125
Calc. mass loss (found) %
32.41 (32.38)
23.45 (23.42)
37.23 (37.21)
12.40 (12.37)
16.33 (15.31)
4.11 (4.08)
Assignment
HL
Loss of phenol
125–205
205–395
395–999
100–190
190–330
330–898
>900
Loss of furan
Loss of C5H4N2O
Loss of carbon atoms
Loss of 2H2O (hydrated) + 1/2Cl2
Loss of H2O (coordinated)
Loss of C18H13N2O
CoO
[Co(L)(H2O)Cl].2H2O
[Ni(L)(H2O)Cl].4H2O
[Cu(L)(H2O)Cl].3H2O
[Zn(L)(H2O)Cl].5H2O
57.69 (57.94)
17.11 (17.09)
22.70 (22.68)
3.80 (3.78)
100–188
170–340
340–899
>900
Loss of 4H2O (hydrated) + 1/2Cl2
Loss of H2O (coordinated)
Loss of C18H13N2O
NiO
57.72 (57.69)
15.77 (15.75)
19.43 (19.41)
3.91 (3.87)
100–189
160–360
360–897
>900
Loss of 3H2O (hydrated) + 1/2Cl2
Loss of H2O (coordinated)
Loss of C18H13N2O
CuO
59.36 (59.32)
17.26 (17.23)
25.19 (25.16)
3.61 (3.57)
100–190
160–340
340–898
>900
Loss of 5H2O (hydrated) + 1/2Cl2
Loss of H2O (coordinated)
54.85 (54.82)
16.33 (16.30)
C
18H13N2O
ZnO

8
ALTURIQI ET AL.
TABLE 8 Kinetic parameters of the furan-Schiff's base ligand (HL) and their corresponding metal(II) complexes
Parameters
Decomposition
Ea
ΔS*
ΔH*
ΔG*
Correlation
coefficient
Compound
temperature (ꢁC)
Method
CR
(kJ/mol−1
)
A (s−1
5.2
)
(J mol−1 K−1
)
(kJ/mol)
(kJ/mol)
.Hl
122–447
142–372
175–315
193–347
187–374
28.4
38.1
28.6
38.9
42.3
41.7
62.9
63.2
62.7
68.8
68.3
69.1
66.6
67.2
66.8
−247
−235
−241
−203
−207
−205
−223
−222
−224
−238
−236
−237
−242
−244
−243
24.2
33.3
31.8
54.2
55.6
56
164
169
165
162
165
163
158
155
157
167
166
164
159
158
157
0.9977
0.9972
0.9974
0.9924
0.9928
0.9929
0.9897
0.9895
0.9898
0.9896
0.9899
0.9897
0.9887
0.9888
0.9885
HM
PN
6.5
5.8
[Co(L)(H2O)Cl].2H2O
[Ni(L)(H2O)Cl].4H2O
[Cu(L)(H2O)Cl].3H2O
[Zn(L)(H2O)Cl].5H2O
CR
162
163
162
172
173
174
180
182
179
169
170
171
HM
PN
CR
63.2
64.8
63.8
54.2
64.8
65.2
64.7
55.8
56.3
HM
PN
CR
HM
PN
CR
HM
PN
CR = Coats–Redfern; HM = Horowitz–Metzger; PN = Piloyan–Novikova.
3.5.5
|
Powder XRD
by 9 compared to that of the free-electron value (2.07) is an
approximate measure of the ligand field strength; the stron-
ger the furan-Schiff's base ligand field, the smaller the
decrease in the g-value, and vice versa.
Single crystals of the corresponding furan-Schiff's base metal
complexes could not be obtained. The cell parameters
(Figure S4; Table 6) were determined using X-ray powder
FIGURE 5 Molecular structure, electron density, HOMO, LUMO, and electrostatic potential of the ligand HL

ALTURIQI ET AL.
9
FIGURE 6 Molecular structure, electron density, HOMO, LUMO, and electrostatic potential of the CoII–L complex
diffraction data. The indexing was carried out using the (CCP4,
UK) CRYSFIRE program[32] for the [CoL(H2O)Cl]ꢀ2H2O,
[NiL(H2O)Cl]ꢀ4H2O, [CuL(H2O)Cl]ꢀ3H2O, and [ZnL(H2O)
Cl]ꢀ5H2O complexes, giving triclinic, monoclinic, orthorhom-
bic, and orthorhombic crystal systems, respectively.
The Co(II), Ni(II), Cu(II), and Zn(II) complexes showed
TG curves in the temperature range ~100–190 C corre-
ꢁ
sponding to the loss of two, four, three, and five hydrated
water molecules + 1/2 Cl2 molecule. The second stage is
related to the loss of one molecule of the coordinated water
for the metal(II) complexes (Co(II), Ni(II), Cu(II), and
Zn(II))\. The third stage is related to the loss of the part of
furan-Schif base ligand (HL). The final weight losses are
due to the decomposition of the rest of the ligand and
metal(II) oxide residue.
3.5.6
| Thermal analysis
The thermal properties of the furan-Schiff's base ligand
(HL) and the corresponding metal(II) complexes (1–4) were
characterized on the basis of thermogravimetric analysis
(TGA) results. The TGA curves are given in Figure 4b, and
the data are listed in Table 7. The weight losses for each
compound were calculated within the corresponding temper-
ature ranges.
3.5.7
| Kinetic studies
The thermodynamic activation parameters (Table 8) of
decomposition processes of the furan-Schiff base metal(II)
complexes, namely the activation energy (E*), entropy
(ΔS*), and Gibbs free energy change of the decomposition
(ΔG*), were evaluated graphically by employing three methods,
that is, those of Coats–Redfern[33] (CR), the Horowitz–Metz-
ger[34] (HM), and the Piloyan–Novikova[35] (PN). From the
results obtained, the following remarks can be made:
The furan-Schiff base ligand (HL) shows four decompo-
sition steps. The first stage occurs in the temperature range
ꢁ
50–125 C and is attributed to the loss of the phenol moiety
(found 32.38% and calc. 32.41%). The second stage in the
ꢁ
temperature range 125–205 C corresponds to the loss of
furan (found 23.42%, calc. 23.45%). The third stage in the
ꢁ
temperature range 205–395 C corresponds to the loss of a
part of the ligand (C5H4N2O) (found 37.21%, calc. 37.23%).
1. The energy of activation (E) increases on going from
one decomposition stage to another for a given complex,
indicating that the rate of decomposition decreases in the
same order. Generally, stepwise stability constants
The fourth stage occurs in the temperature range
ꢁ
395–999 C, corresponding to the loss of carbon atoms
(found 22.1%, calc. 22.00%).

10
ALTURIQI ET AL.
FIGURE 7 Molecular structure, electron density, HOMO, LUMO and electrostatic potential of the NiII –L complex
decrease with the increase in the number of ligands
attached to a metal ion. Conversely, during the decom-
position reaction, the rate of removal of the remaining
ligands will be smaller after the expulsion of the furan-
Schiff's base ligand (HL).[36]
theoretical physical parameters such as bond length, bond
angles, highest occupied molecular orbital (HOMO), and
lowest unoccupied molecular orbital (LUMO) were recorded
using the Accelrys Material Studio.[38]
If any deviations in the bond distance, bond angle, or tor-
sion angle, are evidenced, specific electronic interactions can
be detected, confirming the earlier spectral evidences.[39] Thus,
the physical dimension of the molecules helps to demonstrate
the changes occurring during their topological assemblies.
The deprotonated furan-Schiff base ligand (HL) (Figure 1)
is coordinated as a tetradentate ligand via the carbonyl oxygen,
the two azomethine nitrogen atoms, and the oxygen of the
furan ring. The coordination number of 6 is completed by one
chlorine with one water molecule of the metal ions (CoII, NiII,
CuII, and ZnII).
The molecular structures, along with atom numbering,
electron density, HOMO, LUMO, and electrostatic potential
of the furan-Schiff's base ligand (HL), are depicted in
Figure 5, whereas the same parameters are represented for
the metal complexes in Figures 5–9.
2. The ΔG values increase significantly for the subsequent
decomposition stages because of the increase in the TΔS
values from one stage to another. This may be attributed
to the structural rigidity of the remaining complex after
the expulsion of more ligands, as compared with the pre-
cedent complex, which requires more energy, TΔS, for
its rearrangement before undergoing any compositional
change.[36]
3. The negative ΔS values for the decomposition steps
indicate that all the studied metal(II) complexes are more
ordered in their activated states.[37]
4. The positive ΔH values mean that the decomposition
processes are endothermic.[37]
3.5.8
| Molecular modeling
Comparisons between the bond lengths and bond angles
of the ligand and those of the CoII, NiII, CuII, and ZnII com-
plexes are listed in Tables S1 and S2, respectively. Various
Molecular modeling gives a blueprint of the three-
dimensional arrangement of atoms of any compound. The

ALTURIQI ET AL.
11
FIGURE 8 Molecular structure, electron density, HOMO, LUMO, and electrostatic potential of the CuII–L complex
energetic data were calculated for the furan-Schiff's base ligand
and its metal complexes and are shown in Tables S1 and S2.
According to these data, many conclusions can be drawn:
(10), and N(10)–M(21)–O(18) with different values,
ranging from 64.489ꢁ to 136.574ꢁ.[41]
• The electrostatic potential is the energy of interaction of
a point positive charge with the nuclei and electrons of a
molecule, and its value depends on the location of the
point positive charge (Table 9). Therefore, it is important
to depict the structure of the electrostatic potential of the
ligand and complexes.
• The complexes show octahedral geometry according to
the values of the bond angles formed by the metal ion
and the coordinated groups.
• The higher HOMO energy lies around the phenyl group,
while the lower LUMO energy exists around the furan ring.
• A lower HOMO energy shows that the molecule’s donat-
ing electron ability is weak. On the contrary, a higher
HOMO energy implies that the molecule is a good elec-
tron donor. LUMO energy presents the ability of a mole-
cule to receive an electron.[42]
• The actual bond angles and bond lengths are close to the
optimal values, and thus the proposed structures of the
compounds are acceptable.
• The bond lengths C(6)–N(7), C(9)–N(10), and N(10)–C
(11) of the furan-Schiff base ligand were enlarged in the
complexes as a result of the participation in coordination
upon forming M–N(7) and M–N(10) bonds, whereas C(1) –
O(5), C(4) –O(5), and C(13) –O(18) were mostly shortened
but in some complexes enlarged upon coordination.
• The bond length O(18)–H(30) with the value 0.975 Å
disappears upon coordination, as the ligand acts as a
mono-negative tetradentate by losing this proton. Also,
the value of the bond angle C(13)–O(18)–H(32), which
is 108.184ꢁ, ensures the sp3 hybridization of the oxygen
atom which is an optimal assumption.
• This ligand's bond angles C(1)–O(5)–C(4), C(6)–N(7)–C
(8), C(9)–N(10)–C(11), C(12)–C(13)–O(18), and C(14)–
C(13)–O(18) are reduced or increased upon
complexation.[40]
3.6 | Biological activity studies
3.6.1
| Antitumor activity
The HL ligand and its isolated metal(II) complexes were
tested for antitumor activity against the colon carcinoma cell
• New bond angles were formed between the ligand and
metal ions such as O(5)–M(21)–N(7), N(7 –M(21)–N

12
ALTURIQI ET AL.
FIGURE 9 Molecular structure, electron density, HOMO, LUMO, and electrostatic potential of the ZnII–L complex
TABLE 9 Some energetic data containing total energy, binding energy, dipole moment, HOMO, LUMO, ΔEGap, and electrostatic potential
Total energy
(kcal/mol)
−5.99 × 105
1.040 × 106
−1.057 × 106
−1.075 × 106
−1.095 × 106
Binding energy
(kcal/mol)
Dipole
moment (D)
Electrostatic
potential (kcal/mol)
Compd
Ligand
CoII –L
NiII –L
CuII –L
ZnII –L
HOMO (eV)
−4.863
LUMO (eV)
−2.454
ΔEGap
2.411
1.58
−4285.551
−4659.968
−4651.276
−4585.814
−4561.082
6.1027
9.3964
−1160.707
−4.978
−3.398
−3539.32
10.9206
12.6023
11.1595
−4.931
−3.413
1.518
1.378
1.88
−3319.623
−4.700
−3.322
−1001.360
−5.206
−3.326
−1731.549
TABLE 10 Cytotoxicity for the HL ligand and its isolated metal(II) complexes against human colorectal cancer cells (HCT-116)
Cell viability % at different concentrations (μg)
Compd.
HL ligand
Co(II)
0.0
1.56
100
100
100
3.125
98.64
97.58
96.26
88.78
84.53
6.25
12.50
77.25
75.89
70.12
56.65
72.45
25.00
68.23
62.34
56.56
52.73
59.41
50.00
46.86
42.28
35.69
32.46
39.87
100.0
34.28
32.47
21.88
21.42
26.74
IC50 (μg/mL)a
42.86
100.00
100.00
100.00
100.00
100.00
87.27
86.38
82.16
72.43
85.86
41.12
Ni(II)
28.22
Cu(II)
97.56
94.98
19.37
Zn(II)
17.61
a
IC50 is concentration that can reduce the growth of cancer cells by 50%.
(HCT-116) and the mouse myelogenous leukemia carcinoma
(M-NFS-60) at various concentrations (100, 50, 25, 12.5,
6.25, 3.125, and 1.56 μg/mL). The anticancer efficiency of
all the tested compounds is shown in Tables 10 and 11,
whereas Figures S5 and S6 show the effect of different con-
centrations of all compounds under study on (HCT-116) and
(M-NFS-60), respectively. Additionally, the IC50 values
derived from experimental data are summarized in Tables 10
and 11 and represented graphically in Figures S7 and S8. All
the metal complexes were found to be cytotoxic against
(HCT-116) and (M-NFS-60) and produced 50% cell death in
the concentration range 17.51–42.86 μg/mL. Among the

ALTURIQI ET AL.
13
TABLE 11 Cytotoxicity for the HL ligand and its isolated metal(II) complexes against leukemia cells (M-NFS-60)
Cell viability % at different concentrations (μg)
Compd.
HL ligand
Co(II)
0.0
1.56
98.82
100
98.92
3.125
98.43
97.24
96.14
88.57
84.42
6.25
12.50
77.18
75.68
70.02
56.46
72.32
25.00
68.12
62.23
56.46
52.43
59.34
50.00
46.71
42.18
35.57
32.37
39.63
100.0
34.17
32.36
21.72
21.34
26.62
IC50 (μg/mL)a
100
100
100
100
100
87.14
86.23
82.02
72.33
85.74
42.68
41.03
28.15
19.23
17.53
Ni(II)
Cu(II)
84.26
97.58
Zn(II)
a
IC50 is concentration that can reduce the growth of cancer cells by 50%.
TABLE 12 Antimicrobial screening results of the HL ligand and its isolated metal(II) complexes
Microorganisms
HL ligand
Co(II) complex
Ni(II) complex
Cu(II) complex
Zn(II) complex
Std.
Fungi
Amphotericin B
25.8
Aspergillus Fumigatus (RCMB 02568)
Syncephalastrum racemosum (RCMB 05922)
Geotricum candidum (RCMB 05097)
Candida tropicalis (RCMB 05168)
Gram-positive bacteria
NA
NA
NA
NA
NA
NA
NA
NA
17.8
NA
22.8
NA
20.8
NA
20.7
12.8
NA
20.8
NA
16.8
NA
29.4
23.6
Ampicillin
23.6
Streptococcus pneumonia (RCMB 010010)
Bacillis subtilis (RCMB 010067)
Gram-negative bacteria
NA
NA
NA
5.6
12.2
15.7
16.4
18.4
15.8
17.4
23.5
Ciprofloxacin
22.7
Salmonella typhi (RCMB 010072)
Escherichia coli (RCMB 010052)
NA
NA
NA
NA
NA
NA
NA
13.4
21.2
15.3
25.3
Note: Test was done using the diffusion agar technique; NA: No activity.
TABLE 13 Inhibition efficiencies of the isolated metal(III) complexes on carbon steel corrosion in 0.5 M HCl at 25 ꢁC obtained by weight loss method
Time
Cinh. (1.0 × 10−3 M)
Rate ×10−2
HCl
8.00
—
HL ligand
7.85
Co(II) complex
6.36
Ni(II)complex
6.23
Cu(II) complex
6.78
Zn(II) complex
3.34
2
%Inhibition
3.84
12.45
21.05
19.24
34.21
4
8
Rate ×10−2
6.75
—
5.87
5.21
4.51
4.76
2.97
%Inhibition
Rate ×10−2
9.47
21.45
34.21
24.57
41.71
0.52
—
0.46
0.28
0.29
0.26
0.26
%Inhibition
11.36
39.89
41.32
39.47
55.78
metal complexes, the Zn(II) complexes with IC50 values of
17.61 and 17.53 μg/mL were the most active with the lowest
IC50 value. Additionally, the Cu(II) complex showed compa-
rable effect with IC50 (19.23 and 19.37 μg/mL). The antitu-
mor activity of the compounds follow the order
ciprofloxacin, and amphotericin B as standards. The antibac-
terial and antifungal activities are given in Table 12 and
represented graphically in Figures S9 and S10. The follow-
ing conclusions have been drawn:
HL ligand < Co(II) complex < Cu(II) complex <
Ni(II) complex < Zn(II) complex for both colon carcinoma
cell (HCT-116) and mouse myelogenous leukemia carci-
noma (M-NFS-60).
1. HL
and
its
Co(II),
Ni(II),
Cu(II),
and
Zn(II) complexes possess a broad spectrum of activity
against the sensitive organisms except Syncephalas-
trum racemosum and Salmonella typhi, as they did
not show any activity toward them. Similarly, no
effect was observed for the Co(II) complex toward
any sensitive organisms.
3.6.2
| Antimicrobial activity
The antimicrobial activity of HL ligand and its Co(II),
Ni(II), Cu(II), and Ce(III) complexes was measured toward
two Gram-positive bacteria species, namely Streptococcus
pneumonia and Bacillus subtilis, and two Gram-negative
bacteria species, namely Salmonella typhi and Escherichia
coli, in addition to four fungi species (Aspergillus fumigatus,
Syncephalastrum racemosum, Geotricum candidum, and
Candida tropicalis) in comparison with ampicillin,
2. No effect was observed for the HL ligand toward all sen-
sitive organisms, and such inhibition was enhanced on
complexation especially in case of Ni (II), Cu(II), and
Zn(II) complexes, but less than the standard used.
3. The Zn(II) complex displayed high activity toward all
sensitive organisms except Candida tropicalis and Sal-
monella typhi.

14
ALTURIQI ET AL.
OH2
Ni
O
O
N
4H2O
N
Cl
2
OH2
Cu
O
O
O
N
.2H
CuCl2
2O
OH2
Co
3H2O
OH
N
N
N
Cl
O
O
N
CoCl2.6H2O
2H2O
N
HL
Cl
Z n C l
3
2
1
OH2
Zn
O
O
5H2O
N
N
Cl
4
SCHEME 1 Synthesis of the metal complexes Schiff base-derived from HL with Co(II), Ni(II), Cu(II), and Zn(II) ions
4. The Ni(II) and Cu(III) complexes displayed high activity
toward all sensitive organisms except Syncephalastrum
racemosum, Candida tropicalis, and Salmonella typhi.
5. The antimicrobial activity of the synthesized complexes
is promising; so determining the minimum inhibitory
concentration (MIC) for the synthesized complexes is
recommended.
HCl < HL < Co(II) complex < Cu(II) complex <
Ni(II) complex < Zn(II) complex.
4
| CONCLUSIONS
The geometrical structures of the complexes of the furan-
Schiff base ligand HL with Co(II), Ni(II), Cu(II), and
Zn(II) ions were confirmed by the elemental analyses, IR,
1H, 13C NMR, molar conductance, magnetic moment, UV–
vis, mass spectra, ESR spectra, and thermal analyses data.
Thus, from the IR spectra, it was concluded that furan-Schiff
base ligand HL behaves as a Schiff base tetradentate ligand
with one N2O2 site coordinating to the metal ions via the
two azomethine N groups, the furan O ring, and the deproto-
nated phenolic O group. From the molar conductance data of
the complexes (Λm), it was concluded that the complexes of
furan-Schiff base ligand HL can be considered non-electro-
3.6.3
| Corrosion inhibition
The weight loss method for monitoring the corrosion rate is
useful because of its simple application method and reliabil-
ity. The presence of O and N atoms in the ligand has an
inhibitory effect on steel: we find that it has inhibition of
3.84% after 2 hr and the inhibition effect increases to 9.47%
and 11.36% after 4 and 8 hr, respectively. The Co(II), Ni(II),
Cu(II), and Zn(II) complexes make good inhibitors on steal.
These metals complexes with the HL ligand have an inhibi-
tion effect on steel, as given in Table 13 and in Figure S11.
The inhibition rate follows the order
HCl < HL < Co(II) complex < Cu(II)complex <
Ni(II) complex < Zn(II) complex.
The experiment was repeated after 2, 4, and 8 hr. It
shows that the inhibition rate increases with increasing time
in the ligand and its complexes in the following order:
1
lytes. The H NMR spectra of the free furan-Schiff base
ligand HL shows that the OH signal, which appeared in the
spectrum of furan-Schiff base ligand HL at 11.93 ppm,
completely disappeared in the spectra of its [ZnL(H2O)Cl]ꢀ
5H2O complex, indicating that the OH proton is removed by
the chelation with Zn(II) ion. The octahedral geometry of the
complexes was confirmed using the DFT method from
DMOL3 calculations, UV–vis, magnetic moment

ALTURIQI ET AL.
15
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How to cite this article: Alturiqi AS, Alaghaz A-N,
Zayed ME, Ammar RA. Synthesis, characterization,
biological activity, and corrosion inhibition in acid
medium of unsymmetrical tetradentate N2O2 Schiff
base complexes. J Chin Chem Soc. 2018;1–15.
https://doi.org/10.1002/jccs.201800027